Abstract: Metal-organic
frameworks, high surface area materials with the characteristics of adjustable
structure and surface chemistry, are becoming promising candidates for hydrogen
storage, gas separation, and catalyst supports. In this paper, we explore MOF stability after
various catalytic doping methods for IRMOF-8, Cu-BTC, and Cu-TDPAT. After identifying a catalytic doping technique
that maximizes stability and retains the surface area of the MOF precursor, we
measure hydrogen isotherms at 300K up to 80 bar, and demonstrate catalyst
addition significantly increases hydrogen storage via the hydrogen spillover
effect. Adsorption enhancement is most
pronounced at low pressure, and kinetic limitations and MOF instability effects
inhibit high-pressure adsorption via the spillover effect. Novel hydrogen chemisorption sites are
identified using spectroscopic techniques, for both undoped and doped
MOFs. An improved mechanistic
understanding of the hydrogen spillover effect is developed by tracking of
hydrogenation of N groups via spectroscopy, density functional theory, and
comparison of defected versus pristine MOF structures. Implications for hydrogen storage and use of
MOFs as a catalyst support are discussed.

We utilize density functional theory to explore
hydrogen mobility on doped graphene surfaces, and identify candidate materials
that will meet both thermodynamic and kinetic constraints for room temperature
hydrogen uptake via surface diffusion from a catalyst that dissociates
molecular H2 into active surface species.[1]The results
help to explain recent spectroscopic evidence of reversible ambient temperature
hydrogenation of oxidized carbon surfaces via hydrogen spillover from platinum
nanoparticles[2] and clarify the hydrogen spillover mechanism. The
identified kinetic and thermodynamic constraints demonstrate that significant
mobility at room temperature will occur only via H diffusion in a chemisorbed
state, and this requires heteroatoms or chemical dopants to simultaneously
increase the binding energy and decrease the barrier for chemical diffusion. Despite
prior assumptions in the literature, the binding energy of atomic hydrogen on a
surface is not correlated to mobility when the surface does not directly
dissociate the H2.Beyond
hydrogen storage, clarification of the mechanism by which hydrogen diffuses on
carbon extends to basic surface science, catalysis, astrophysics, novel
materials, energy storage devices, and electronics.

We identify a molecular fingerprint to probe H mobility on catalyzed carbon surfaces and confirm a weak carbon-hydrogen chemical bond may form reversibly at ambient temperature.1We elucidate surface properties that lead to high mobility, and demonstrate reversibility requires mobility back to the catalyst.Our technique extends prior ex post facto evidence of hydrogen spillover to carbon materials, by probing the carbon-hydrogen bond in situ, at high pressure and ambient temperature.The results clarify a mechanism that has been disputed in recent years, as experimental reports claiming combined ambient temperature reversibility and mobility seem to defy theoretical predictions of the nature and strength of the carbon-hydrogen bond and have not been easily substantiated. Beyond hydrogen storage, clarification of the mechanism by which hydrogen diffuses on carbon extends to basic surface science, catalysis, astrophysics, novel materials, energy storage devices, and electronics.

Abstract: Hydrogen
spillover involves addition of a catalyst to a high-surface area microporous
support, such that the catalyst acts as a source for atomic hydrogen, the
atomic hydrogen diffuses from the catalyst to the support, and ideally, the
support provides a high number of tailored surface binding sites to maximize
the number of atomic hydrogens interacting with the surface.Hydrogen spillover has been proposed as a
means to increase the operative adsorption temperature of nanoporous materials from
cryogenic conditions to near ambient temperature.However, this proposition has become highly
controversial in the past few years, due largely to discrepancies between
laboratories, and even variations of the magnitude of hydrogen uptake observed
for materials prepared with near-identical techniques within the same
laboratory.These discrepancies have
pointed to the fact that the hydrogen spillover mechanism is not understood on
a molecular level.Amidst this
controversy, a combined approach of in
situ spectroscopic techniques and theoretical multi-scale modelling
calculations are being used to resolve the hydrogen spillover mechanism and
illuminate the nature of the exact surface sites and structures responsible for
the high uptake in select materials. The first direct spectroscopic evidence of
a reversible room temperature carbon-hydrogen wag mode, and how this
experimental data was used to modify model chemistry in density functional
calculations, will be discussed. The ultimate goal of this project is to not
only resolve the hydrogen spillover controversy, but to use the findings to
design new materials for hydrogen storage and catalytic hydrogenation.

And just for fun.... I included the following picture of my non-research activities in Crete.View image

Professor Angela Lueking has been awarded a highly competitive Marie Curie International Incoming Fellowship to partner with researchers at the University of Crete to study basics of hydrogen adsorption and diffusion on surfaces. The work will include design of novel materials. The abstract for the successful proposal is included below.

Objectives and
Overview:The objective of the proposed work is to
synthesize catalyzed nanoporous materials that have superior hydrogen uptake
between 300K and 400K and moderate pressures (20-100 bar) via the hydrogen
spillover mechanism. Hydrogen spillover involves addition of a catalyst to a
high-surface area microporous support, such that the catalyst acts as a source
for atomic hydrogen, the atomic hydrogen diffuses from the catalyst to the
support, and ideally, the support provides a high number of tailored surface
binding sites to maximize the number of atomic hydrogens interacting with the
surface.The proposed work will provide
a means to explore an extended collaboration to combine in situ spectroscopic techniques and theoretical multi-scale modelling
calculations. Both carbon-based and microporous metal-organic framework (MMOF) materials
with added hydrogen dissociated catalysts will be drawn from past and on-going
projects, in order to identify specific binding sites that lead to appreciable
uptake.First, preliminary spectroscopic
data will be used to validate and extend existing theoretical models. In situ characterization of materials
with systematic variations in structure and/or synthesis will be used to
identify properties that lead to high uptake, including effect of structure,
geometry, surface chemistry, and catalyst-support interface. Resulting
spectroscopic data will be analyzed with theoretical models to conclusively
identify the nature of the binding site. Validated models will be used to
direct future synthesis of novel materials.The overall goal will be to identify tailored surface sites that
reversibly bind atomic hydrogen between 300 K and 400K.

The
work is incredibly timely, as the hydrogen spillover mechanism has become
highly controversial in the past two years, due largely to discrepancies
between laboratories, and even variations of the magnitude of uptake observed
for materials prepared with near-identical techniques within the same
laboratory.Amidst this controversy, a combined approach
of in situ spectroscopic techniques
and theoretical multi-scale modelling calculations will resolve the hydrogen
spillover mechanism and illuminate the nature of the exact surface sites and
structure responsible for the high uptake in select materials. The proposed
work extends previous work of Professor Angela Lueking, who as a graduate
student, was first author on the first papers identifying hydrogen spillover as
a means to achieve appreciable uptake at room temperature. Subsequently, Lueking has studied hydrogen
uptake and adsorption in other materials, and furthered her experience in
material characterization. She has recently returned to the field of hydrogen
spillover, employing in situ spectroscopic
techniques,5, 6 as outlined below.Lueking will pair with George Froudakis of
the University of Crete, whose theoretical calculations (with George
Psofogiannakis, a current Marie Curie fellow) provided the first multi-scale modelling
of the hydrogen spillover mechanism.The proposed work will
provide a means to explore an extended collaboration to combine their
respective work in experiment and theory.The combined approach is expected to not only resolve what has become a
highly controversial issue in the literature, but ultimately, identification of
the key sites responsible for high uptake in select materials is expected to
lead to a significant increase in capacity and reproducibility in hydrogen
spillover materials that are optimized for near-ambient temperature adsorption.